1. Technical Field
The present invention generally relates to a natural gas engine lubricating oil composition and a method for preventing or inhibiting exhaust valve seat recession in natural gas fueled internal combustion engines.
2. Description of the Related Art
Natural gas fueled engines are engines that use natural gas as a fuel source. Lubricating oils with high resistance to oxidation, nitration and viscosity increase are generally preferred for lubricating oils used in natural gas engines because of the conditions related to this type of engine.
Natural gas has a higher specific heat content than liquid hydrocarbon fuels and therefore it will burn hotter than liquid hydrocarbon fuels under typical conditions. In addition, since it is already a gas, natural gas does not cool the intake air by evaporation as compared to liquid hydrocarbon fuel droplets. Furthermore, many natural gas fueled engines are run either at or near stoichiometric conditions, where less excess air is available to dilute and cool combustion gases. As a result, natural gas fueled engines generate higher combustion gas temperatures than engines burning liquid hydrocarbon fuels. In most cases, natural gas fueled engines are used continuously at 70 to 100% load, whereas an engine operating in vehicular service may only spend 50% of its time at full load.
This condition of running continuously near full load places severe demands on the lubricant. For example, by subjecting the lubricating to a sustained high temperature environment, the life of the lubricant is often limited by oil oxidation processes. Also, since the rate of formation of nitrogen (NOx), increases exponentially with temperature, natural gas fueled engines may generate NOx concentrations high enough to cause severe nitration of lubricating oil.
Good valve wear control is also important for keeping engine operating costs down and may be achieved by providing the proper amount and composition of ash. In addition, minimizing combustion chamber deposits and spark plug fouling are considerations in setting the ash content in these oils. Lubricating oil ash levels are limited, so detergents must be carefully selected to minimize piston deposits and ring sticking.
Valve wear resistance is important to the durability of natural gas fueled engines. In general, exhaust valve recession is wear which occurs at the valve and valve seat interface and is the most pronounced form of valve wear in natural gas fueled engines. When the valve is prevented from seating properly, it can cause engine roughness, poor fuel economy and excessive emissions. In order to correct excessive valve wear, a cylinder head overhaul is usually required. Although natural gas fueled engines typically use very hard corrosion-resistant material for the valve face and seat mating surface to give extended cylinder head life, it does not completely eliminate valve recession.
There is a difference in the lubricating oil requirements for natural gas fueled engines and engines that are fueled by liquid hydrocarbon fuels. The combustion of liquid hydrocarbon fuels such as diesel fuel often results in a small amount of incomplete combustion (e.g., exhaust particulates). In a liquid hydrocarbon fueled engine, these incombustibles provide a small but critical degree of lubrication to the exhaust valve/seat interface, thereby ensuring the durability of both cylinder heads and valves.
Natural gas fueled engines burn fuel that is introduced to the combustion chamber in the gaseous phase. The combustion of natural gas fuel is often very complete, with virtually no incombustible materials. This has a significant affect on the intake and exhaust valves because there is no fuel-derived lubricant such as liquid droplets or soot to aid in lubrication to the exhaust valve/seat interface in a natural gas fueled engine. Therefore, the durability of the cylinder head and valve is controlled by the ash content and other properties of the lubricating oil and its consumption rate to provide lubricant between the hot valve face and its mating seat. Too little ash or the wrong type can accelerate valve and seat wear, while too much ash may lead to valve guttering and subsequent valve torching. Too much ash can also lead to loss of compression or detonation from combustion chamber deposits. Consequently, gas engine builders frequently specify a narrow ash range that they have learned provides the optimum performance. Since most gas is low in sulfur, excess ash is generally not needed to address alkalinity requirements, and ash levels are largely optimized around the needs of the valves. There may be exceptions to this in cases where sour gas or landfill gas is used.
Zinc dialkyldithiophosphates are a very effective additive used in natural gas engine oil additive packages for anti-wear and oxidation protection and, for example, has been shown to contribute to the ash formed on the exhaust valve. However, it is believed that zinc dialkyldithiophosphates may actually chemically react with the surface or form a compound with the other sources of ash that can be easily removed from the valve surface.
In addition, a problem associated with the use of zinc dialkyldithiophosphate is that their phosphorus and sulfur derivatives poison the catalyst components of the catalytic converters. This is a major concern as effective catalytic converters are needed to reduce pollution and to meet governmental regulation designed to reduce toxic gases such as, for example, hydrocarbons, carbon monoxide and nitrogen oxides, in the internal combustion engine exhaust emissions. Such catalytic converters generally use a combination of catalytic metals, e.g., platinum and metal oxides, and are installed in the exhaust streams, e.g., the exhaust pipes of automobiles, to convert the toxic gases to nontoxic gases. Accordingly, it would be desirable to eliminate the amount of zinc dialkyldithiophosphate in lubricating oils, thus reducing catalyst deactivation and hence increasing the life and effectiveness of catalytic converters while also meeting future industry standard proposed phosphorus and sulfur contents in the engine oil. However, simply decreasing the amount of zinc dialkyldithiophosphate presents problems because this necessarily lowers the anti-wear properties and oxidation inhibition properties of the lubricating oil. Therefore, it is necessary to find a way to retain the anti-wear and oxidation properties of the engine oils.
U.S. Pat. No. 3,798,163 (“the '163 patent”) discloses a method for controlling or inhibiting exhaust valve recession in natural gas fueled internal combustion engines by maintaining a lubricating amount of a lubricating oil composition on the engine components of the internal combustion engine. The '163 patent further discloses that the lubricating oil composition contains (a) a major amount of an oil of lubricating viscosity, (b) at least one alkaline earth metal sulfonate in an amount sufficient to improve the detergency of the composition, and (c) at least one alkaline earth metal salt of a condensation product of (i) an alkylene polyamine, (ii) an aldehyde, and (iii) a substituted phenol, wherein the alkaline earth metal salt of the condensation product is present in an amount sufficient to inhibit the recession of the engine's exhaust valves into the engine cylinder head.
U.S. Pat. No. 5,726,133 (“the '133 patent”) discloses a low ash gas engine oil comprising a major amount of a base oil of lubricating viscosity and a minor amount sufficient to contribute a sulfated ash content of about 0.1 to 0.6% ash by ASTM D 874 of an additive mixture comprising a mixture of detergents comprising at least one first alkali or alkaline earth metal salt or mixture thereof of low Total Base Number (TBN) of about 250 and less and at least one second alkali or alkaline earth metal salt or mixture thereof which is more neutral than the first low TBN salt. The '133 patent further discloses that the fully formulated gas engine oil can also typically contain other standard additives known to those skilled in the art, including anti-wear additives such as zinc dithiophosphates, dispersants, phenolic or aminic antioxidants, metal deactivators, pour point depressants, antifoaming agents, and viscosity index improvers.
U.S. Pat. No. 6,174,842 (“the '842 patent”) discloses a lubricating composition containing (a) a major amount of lubricating oil, (b) an oil-soluble molybdenum compound substantially free of reactive sulfur, (c) an oil-soluble diarylamine and (d) an alkaline earth metal phenate. The '842 patent further discloses that the composition can further include a zinc dihydrocarbyl dithiophosphate as an anti-wear agent. In addition, Oil Blend 18 disclosed in Example 2 of the '842 patent contained an anti-wear agent and was evaluated for exhaust valve recession in a Cummins Natural Gas Engine test.
It is desirable to develop improved natural gas engine lubricating oil compositions which can prevent or inhibit exhaust valve recession in natural gas fueled internal combustion engines employing a lubricating oil composition substantially free of at least any zinc compound.